Patterns of diversity Flashcards
Species area curves
- what do they describe?
how the number of species recorded increases as the area of habitat investigated increases.
Why do large islands have more species than small islands?
o Positive richness-area correlation is universal pattern (and applies to mainlands too)
o Three main mechanisms have been proposed
• The habitat diversity hypothesis
• The passive sampling hypothesis
• The equilibrium model of island biogeography (=‘area per se’)
o These explanations are not mutually exclusive, but emphasise different processes
o Students should explain these hypotheses clearly an appropriately (equilibrium theory is best explained using diagrams showing rates of immigration and extinction in relation to species richness)
o Good essays will consider the evidence for and against each explanation, from observations and experiments
Give 2 examples of ‘patterns in diversity’ on a macro-ecological scale and suggest the processes that might underpin one of those patterns
Macroecology = branch of ecology that investigates relationships between organisms and their environment on a LARGE-SCALE (across earth’s surface)
i.e. large-scale trends in the distributions of individuals and species
Patterns in diversity on a macro-ecological scale
- species-area relationships (how the number of species recorded increases as the area studied increases)
- latitudinal gradients in species richness (The trend towards increasing diversity in
the tropics (towards the equator))
Macroecological hypotheses tends to be tested using compilations and analyses of OBSERVATIONAL data rather than experiments
SA relationships hypotheses
- habitat diversity is the driver of species-area relationships rather than equilibrium model ‘area per se’
- could be tested by compiling and analysing data for species richness on islands that vary in area and habitat diversity and trying to control for the effects of each explanatory variable statistically eg. mangrove islands OR
- standardisation (=do we see a SAR for islands that vary in area but not in habitat diversity or vice versa).
What are species–area curves?
What equation describes the relationship?
Species–area curve describes the relationship between the area of a habitat and the number of species found within that area.
The closest thing to a universal rule in ecology!!
As you survey increasing areas, the
number of species recorded increases
The increase is not linear – it often follows the relationship S = cA^z
The relationship is linear on a log log graph!!
LogS = Logc + zLogA
S is the number of species
A is the habitat area
z is the slope of the species-area relationship in log-log space
c is a constant - the no. species that would exist if the habitat area was confined to one square unit
Give an example of a species-area relationship and how you would go about plotting the relationship to show it’s there
Butterflies in the Caribbean islands!
• Go around islands, count up species richness on each island
o Calculate species richness as a function of area
o Plot on log-log axis, get characteristic straight line (LogS = Logc + zLogA)
Name the 3 processes that explain species-area curves
Three main mechanisms proposed - They are not mutually exclusive, but
emphasise different processes
- Habitat diversity hypothesis
- Habitat diversity will scale with area (mountain, mangrove, beaches)
- As habitat diversity increases, so does species richness - Passive sampling hypothesis
a. Species accumulate in proportion to habitat area - Equilibrium model of island biogeography (=area per se)
a. Balance between colonisation and extinction
• These mechanisms are not mutually exclusive, but emphasise different processes
Name the 3 processes that explain species-area curves
They are not mutually exclusive, but
emphasise different processes
- Habitat diversity hypothesis
- Passive sampling hypothesis (null model that it’s simply area responsible)
- Equilibrium model of island biogeography (taking colonisation and extinction into account)
Does the Habitat diversity hypothesis explain species-area curves?
- how well?
- failings?
Habitat diversity will scale with area (mountain, mangrove, beaches) because if there is a wider range of habitat types, a wider range of species can live in an area
• Predicts that larger areas have more habitats, and that species diversity increases with habitat diversity
• Support for this hypothesis where area/habitat diversity can be separated
TO TEST, EITHER
o Keep habitat constant and vary area within sampling sites, OR
o Keep area constant and vary habitats within sampling sites
• Habitat diversity often but not always explains patterns
o Explains large proportion in species richness variation with area
Does the Passive sampling hypothesis explain species-area curves?
- how well?
- failings?
- Species accumulate in proportion to habitat AREA (rather than diversity!)
• Often used as a ‘null model’
Assumes
o The probability that an individual or a species occurs on an island is proportional to island area
o Islands sample individuals randomly and independently
Failings…
Fails to explain the diversity on small islands
o Given enough time, all species should be present on small islands – nothing limits diversity
o Does not predict turnover of species (does not suggest we should lose species through time)
What is the Equilibrium model of island biogeography
- how does it attempt to explain species-area curves?
- tested by
MacArthur & Wilson (1963, 1967): island biogeography theory
Proposes that the no. species found in an undisturbed insular environment (“island”) is determined by immigration and extinction, and that rates of immigration and emigration are affected by the distance of an island from a source of colonists (mainland)
o Takes into account the balance of rates of colonisation and extinction
o Considers size area of island, as well as isolation of the island
- Closer islands will have more species than isolated islands
- Large islands will have more species than small islands
BEST explains species richness on oceanic islands
Tested by
- Simberloff & Wilson (1969) classic experiment with mangrove islands
- Diamond (1969) studied birds on the Channel Islands archipeligo
On an island, what happens to the rates of immigration and extinction as no. species increases?
As number of species on the island increases
Rate of immigration from the mainland (source) decreases as the no. potential colonists from the mainland is decreasing (they’ve already colonised the island!!)
Rate of extinction increases as more species = more competition
These lines are CURVED not straight in more realistic models -
• Strong dispersers colonise easily, then rates of colonisation slow
• Extinction rate is slow initially - many empty niches - but increases as the island becomes saturated with competing species
What’s Š?
What does Š depend on?
Š is the equilibrium no. species where immigration replaces the number of species going extinct. Any shift from the equilibrium is driven back towards it.
Depends on area of island and isolation
(small pops on small islands are more prone to extinction, distant islands harder to reach so less immigration)
Assumptions and predictions of the Equilibrium model of island biogeography
Assumptions:
• Evolution does not influence species richness (no in-situ speciation)
• Isolated islands are less likely to be colonised
• Big islands have big populations
• Big populations are less likely to go extinct (SPP!)
• Colonisation and extinction is independent of species composition on the island - This one’s not likely to be upheld because competitor species may affect colonisation of a species
Predictions:
o There should be substantial TURNOVER in species COMPOSITION, but species NUMBER should be relatively CONSTANT . through time
o All else being equal the number of species present on an island should decrease with increasing isolation
Who tested MacArthur’s ‘Equilibrium model’ and what did they conclude?
Both concluded significant species turnover but constant species richness, upholds MacArthur’s theory
Simberloff & Wilson (1969) classic experiment with mangrove islands
- Fumigate islands with fumigated with methyl bromide insecticides to clear their arthropod communities
- survey species richness and composition change over time
- closely monitor the immigration of species onto the islands
o Within two years, species richness on mangrove islands recovered to similar levels as before fumigation due to immigration by colonisation
o However, species composition v diff to before fumigation (only 7 to 28% similarity), and diff between islands
o Also, islands closer to the mainland recovered faster as predicted by the Theory of Island Biogeography. The effect of island size was not tested, since all islands were of approximately equal size.
Diamond (1969) studied birds on the Channel Islands archipeligo (California)
- Looked at colonisation and extinction rates on the different islands
- The no. species present was quite
constant but their identities changed (same as above)
What about truly isolated islands?
- tended to have fewer species!
- experiments of island isolation also uphold the assumption that isolated islands have lower species richness
eg. Bahama island birds
How are mainlands different to islands?
Species-area curves apply to both
Mainland
‘ a self-contained region where species
originate entirely by speciation within
the region’
- slope z tends to be lower with log-log species-area curves (less increase in species richness per unit area)
- however, massive mainlands like continents actually have higher z values due to being so big that they have low extinction and high speciation rates (this drives the area hypothesis for high diversity in the tropics)
Island
- colonisation by immigration from the mainland is very important eg. to speciation
- slope z tends to be higher with species-area curves
Why is the z slope lower with species-area curves on mainlands compared to islands?
• Think of an island of 1 ha compared with an area of mainland of 1 ha
• On the mainland, many species will be present as ‘sink’ populations (not self-supporting, NEED a source)
o Species present in area due to supporting population around them
• On the island these would go extinct
o 1 ha not large enough to support some of the species
• This tends to slow the rate at which new species are encountered as you increase area on the mainland
Mainland patterns
- what’s responsible for the species-area relationship?
Different processes contribute
Artefact?
o Small areas may have been more thoroughly censused than large areas
o But this explanation only produces shallow slopes
More geographical provinces?
o But most measured within one province
o This is important for small islands
Habitat diversity? YES
o area is correlated with habitat diversity, and when habitat diversity is held constant the species–area relationship disappears!!
• If look at different continents, z values increase again as there is sufficient space for radiations
How can species-area curves be used in conservation?
- important assumptions & drawbacks
Over 30 years of use for conservation planning
o You can predict how many extinctions will occur following area reduction
o AND suggest the best ways of slowing the loss (reserve design)
Remember that species richness is the primary object of conservation interest - NOT conserving a particular species
Important assumptions and drawbacks
Species-area graphs assume…
• loss of species results directly from habitat area reduction
o BUT many other factors may be at play e.g. fragmentation, invasive species
• species are distributed homogeneously
o But species vary in the size of their distributions, and the extent to which these overlap
• slope of species-area curve accurate and constant
o BUT species-area curves vary with scale of measurement. Predictions depend critically on the slope of the line so you really need to be confident in z value
These all lead to inaccuracies in the predicted effects of habitat loss
How do z values vary at different spatial scales?
Mainland vs island
- On a mainland you can find sink populations (cannot sustain themselves without the ‘income’ of migration) which would go extinct if they were on an island.
- Therefore these sink populations ‘blur the distinctiveness’ between different areas of mainland (kind of homogenise it) so reduce the frequency at which you encounter new species as you sample more area of mainland, giving it a lower z value
However, with continents, they’re SO big that they’re basically like loads of islands joined together as there is sufficient space for radiations, so they actually have a high z value
Draw a labelled diagram to show how a species-area curve can be used to predict the number of species that might go extinct following habitat destruction.
- drawback
y axis = no. species
x axis = area
There’s a line on the graph (straight in lectures but shouldn’t it be the top bit of a c?? - maybe above are both log)
Draw two horizontal lines up from area and go left across to correlating no. species
Can use to determine if you lose X amount of area no. species Y you will lose
Drawback - Predictions depend critically on the slope of the line so you really need to be confident in z value
An example of where species-area curves work very well in conservation
- assumptions
Example: Forest birds in eastern North America
• 52% of forests were cleared in last 200 years
• Considering species that occur only in that habitat (i.e. that are endemic), species-area relationship predicts 4.8 species lost in this time period
o This estimate is v close to what is seen in reality.. 4 have gone extinct, with a fifth very close to extinction
o Extent of forest loss closely matches magnitudes of extinction
Assumptions
• Slope of species area curve accurate and constant
o BUT species-area curves vary with scale of measurement
• Loss of species results directly from habitat reduction
o BUT many other factors may be at play e.g. fragmentation, invasive species
• Species are distributed homogeneously
o But species vary in the size of their distributions, and the extent to which these overlap
These all lead to inaccuracies in the predicted effects of habitat loss
Extinction risk from climate change
- what % of species are ‘committed to extinction’? How is this calculated?
- An example of a species extinct because of climate change
Adding to Jared Diamond’s ‘evil quartet’…
Climate change has and will cause shifts in distributions
Look at changes in the ‘climate envelope’ for each species
o Shift to remain in viable climatic ranges, might be shrinking or expanding
Assume that a species either has no limits to dispersal or that it is incapable of dispersal
o But: species may not be able to realise the available distribution, or not be able to distribute due to habitat fragmentations
Use the climate envelope model to project its future distribution
o The new distribution is the overlap between current and future potential distributions
Predict, on the basis of mid-range climate-warming scenarios for 2050, that 15–37% of species will be ‘committed to extinction’
o Can species be assumed to track the climate envelope?
EG. Golden toad due to extreme El Nino event exacerbated by climate change
Designing nature reserves.. should you have a Single Large reserve Or
Several Small ones of a similar area ? (SLOSS controversy)
- why the debate is shit
Size matters
o Larger areas have more species
o Larger should be better, maximise number of species protected in the area
However, depends on degree to which species overlap between areas
o Sometimes, many small islands can collectively have more species than an equivalent, large area
Why the debate is shit
- This debate about ‘designing’ reserves is often irrelevant because we can rarely choose between one large versus several small reserves!!
- Too much focus on extinction and . insufficient on colonisation
o Ignore important processes. For example, many species can persist in networks of habitat patches through repeated recolonisation (metapopulations) - Ignores important issues about managing reserves to minimise extinctions
Are species-area relationships and island biogeography theory of use to conservationists? Discuss
YES
o You can use curves to predict how many extinctions will occur following area reduction
o AND from that suggest the best ways of slowing the loss (reserve design)
Eg. Forest birds in eastern North America
• 52% of forests were cleared in last 200 years
• Considering species that occur only in that habitat (i.e. that are endemic), species-area relationship predicts 4.8 species lost in this time period
o This estimate is v close to what is seen in reality.. 4 have gone extinct, with a fifth very close to extinction
o Extent of forest loss closely matches magnitudes of extinction
NO
There are lots of assumptions and drawbacks.
Species-area graphs assume…
• loss of species results directly from habitat area reduction
o BUT many other factors may be at play e.g. fragmentation, invasive species
• species are distributed homogeneously
o But species vary in the size of their distributions, and the extent to which these overlap
• slope of species-area curve accurate and constant
o BUT species-area curves vary with scale of measurement. Predictions depend critically on the slope of the line so you really need to be confident in z value
These all lead to inaccuracies in the predicted effects of habitat loss
What is latitudinal gradient in species richness?
- give examples
The diversity of almost all plant and animal groups increases as we move from the poles towards the equator.
IMPORTANCE OF TROPICS
• At least 50% of the earth’s species are tropical!!
• The habitats with greatest biodiversity on the LAND and in the SEA are both tropical
Tropical rainforests
• Cover 7% of the earths surface
• Account for >50% of the earth’s biodiversity
Tropical Coral Reefs
• Cover 1% of the earth’s surface
• Account for 25% of all marine fish species
VERY CLEAR RELATIONSHIP BETWEEN LATITUDE AND SPECIES RICHNESS (incr. latitude dec. no. species)
eg. Land - Mammals in NA
eg. Sea - Coastal marine fish
Has numerous competing explanations
Why study latitudinal gradients?
Why are tropics so diverse? is a long-term fascination for biologists
• Catalyst for much ecological theory
• Relevant to conservation as…
o The greatest threats to biodiversity are where diversity is the highest! We want to conserve these areas
o LGs may affect spread of invasive species, the control of diseases and their vectors, likely effects of global climate change on biodiversity
▪ Eg ebola distribution changes across LGs
Exceptions to the latitudinal gradient macro-ecological pattern in diversity
There are a few exceptions,
o Penguins, do not peak near the equator
o Ichneumonid wasps, same in tropical and temperate
o But these tend to be individual clades with quirky patterns of diversification
How robust is the latitudinal gradient macro-ecological pattern in diversity
- How long has it been around?
- Does it hold at all levels of taxonomic hierarchy and spatial scales?
The pattern is ancient (at least 70 x 10^6
yrs)
The pattern holds at different levels in the taxonomic hierarchy – eg. species, genera, family and order level for birds
The pattern holds at various spatial scales
o Broad climatic zones
▪ E.g. ‘polar’ ‘temperate’ and ‘tropical’
o Assemblages in geographic subdivisions
▪ E.g. latitudinal bands
o Local ecological communities
▪ E.g. 1 km2 plots
Even when you control for area (because the width of continents is not constant at all latitudes) you still see the pattern
NOTE: species richness differences towards the south due to size narrower sizes of continent, not latitude!! (it holds up:))
Name the explanations for latitudinal gradients
Many competing explanations
- Can be categorised into biotic and abiotic…OR by the way in which the available resources are divided among species
‘Biotic’ explanations,
o Productivity/ species energy
o Competition/ species specialisation (niches & dd)
o Predation/ overlap in resource use
‘Abiotic’ explanations
o Time & Stability/ full exploitation of resources
The explanations are not mutually exclusive
Explain the Productivity/ Species Energy
Hypothesis for latitudinal gradients
The hottest, most humid places (nearest to the equator, with lowest latitude) have the greatest net primary productivity
This is because they have a greater range of possible resources
• The greater the range of resources, the more that are large enough to support individual species
BUT…
– Tropical seas have low productivity but high species richness
– Eutrophic lakes have high productivity but low richness
AND
High productivity does not necessarily lead to high biomass/abundance anyway
Explain the competition/ specialisation
hypothesis for latitudinal gradients
NEGATIVE DENSITY DEPENDENT SURVIVAL
In the tropics, each species is more specialised and exploits a smaller ranges of resources. This means you can pack more in in a given area.
• In the tropics, climate is constant
• Most species grow toward their carrying capacity (due to lack of perturbations in environment??)
o This means there is intense competition for resources
o This causes narrowing of niche breadth
o We know from MacArthur’s study on Cape May warblers in trees that having a niche reduces density dependence and stabilises dynamics
BUT
• Comparative analysis of strength of competition and niche breadth is difficult
And
• However there’s little evidence that specialisation and density dependence varies latitudinally
o So even though it’s true, it can’t explain why more species are coexisting in the tropics
Eg. Host specificity of folivorous (leaf-eating) insects
• Graph of no. species vs no. hosts
• Although there are more species in the tropics, the shape of the distribution is the same in tropics and temperate
o If more species were more specialised in the tropics, the 1st bar would be wayyy bigger than the rest of the bars in the tropics (1 host would give rise to disproportionately more species in the tropics). This is not seen.
Explain the predation/ overlap in resource use hypothesis for latitudinal gradients
In the tropics there’s more overlap in resource use due to predation, parasitism, and herbivory
Natural enemies (predators) may promote diversity and coexistence!!
• Tropical climate is stable, so more intense mortality from ‘natural enemies’ rather than death by environmental perturbations
o This means pop sizes of their prey are kept low
o Reduces competition
o Allows overlap in resource use and prevents any one species becoming locally dominant
• This means many species can coexist in the tropics
E.g. Coexistence of tropical forest trees
o 100s of species per hectare.
o Why?
• Natural enemy impact particularly severe
o Seedling mortality is strongly dd. within species ie. seedlings do well where they are rare, and poorer where they are common
• Important role for pathogens and herbivores
Janzen-Connell Effect for ‘POSITIVE DISTANCE-DEPENDENT SURVIVAL’
o Growing close to parent tree is really hard as specialised pathogens and herbivores kill/eat seedlings
o PRC = population recruitment curve
▪ Sweet-spot of establishment
o Prevents any one species from becoming locally dominant
▪ Other species have opportunity to coexist, stabilising populations and giving high diversity!!
Explain the time and stability hypothesis for latitudinal gradients
Resources are more fully exploited in the tropics because they are more stable (unchanged over time or season) and speciation appears to happen faster
Communities diversify with time
- temperate regions have younger communities due to glaciation, whereas
- tropics relatively unchanged for many many years
Tropics are also stable in the sense of low seasonality
• Empty niches more widespread in temperate areas, and there’s no time for species to re-invade (ecological time) or evolve to fill them (evolutionary time)!! before shit changes
BUT
o Many exceptions where stable habitats have low diversity, e.g. tropical mountain tops
o Many tropical environments DO have seasonally fluctuating environments
ALSO
• Faster speciation in the tropics
– Higher rates of mutation (temperatures??)
– Shorter generation times
– More intense selection (predators, interspecific competition)
BUT little direct evidence on speed of
evolution
Considering all hypotheses, what’s the answer to why there are latitudinal gradients in species richness?
- why is it hard to come to a conclusion?
With few exceptions, diversity decreases as we move toward the poles.
- There are many plausible biotic and abiotic explanations (Productivity, Competition, Predation, Time & Stability) – each has its problems
- The explanations are not mutually exclusive
- Both ecological and evolutionary factors need to be accounted for (‘generation’ of diversity vs. maintenance of diversity)
- And any explanations will need to take into account area (Rosenzweig 1995) and geometric (Colwell and Hurtt 1994) constraints
The search for a single ‘cause’ may be futile
Explaining the gradient is difficult because
• Many of the explanations are circular
o Need diversity to drive the mechanism that elevated diversity
• Most of the explanations make (similar) qualitative, not quantitative predictions
o Don’t shape the relationship between latitude and species richness
o If they describe the shape of the relationship we could tell which model best fits it
• The data are often messy (many sources)
• Our sample size is small (n = 1)
o Can’t look at other planets
• Doing experiments is difficult
Explain Rosenzweig’s ‘Area’ hypothesis for latitudinal gradients
Recent hypothesis - Null model
Michael Rosenzweig notes that species–area relationships for very large areas behave differently from species–area relationships from islands or smaller contiguous areas (the continent idea- so big it allows radiations just like islands so have high z value!!)
This applies to the tropics!!
- The tropics cover a greater area than any other major ecological zone (‘biome’) (because the two tropics are next to each other, so their area is double the poles that are separated)
- Average tropical species has geographical range about 5x larger of temperate
The no. species present is a balance between speciation rates and extinction rates (No. Species present = Species Evolving – Species going extinct)
This large area may mean tropics have higher speciation rates and lower extinction rates as function of number of species, which leads to v high diversity
• Lower extinction rates because of
- larger pops and refuges from environmental change
• Increased speciation rates because of
o Geographical barriers allowing allopatric speciation
o Greater chance of a rare mutation (more rapid evolution)
This could account for the huge diversity seen in the tropics (latitudinal gradient)!
However… AREA CANNOT ALONE EXPLAIN all aspects of the latitudinal gradient, e.g. low diversity of deserts, arctic tundra
AND it may be misleading to think of the tropics as a single uniform ‘biome’
Explain Colwell & Hurtt’s ‘Geometric constraints’ hypothesis for latitudinal gradients
Recent hypothesis - Null model
Colwell & Hurtt (1994) showed that you can expect a latitudinal gradient because of the ‘boundedness’ of the earth
• There are northern and southern boundaries to where species can exist
The mid-domain effect!!
• If you place species at random on the earth, within these boundaries, the range of species will overlap much more in tropics
• Doesn’t involve any biology
• Different assumptions, e.g. about how to randomly place ranges, affect quantitative results but not overall pattern
• Makes quantitative predictions (easier to test than many other hypotheses)
• Fits empirical data well but not perfectly
ALSO NOT THE SOLE EXPLANATION
